1. Field of the Invention
This invention relates to a photovoltaic element using a non-monocrystalline semiconductor, and a method of and an apparatus for continuously forming the non-monocrystalline semiconductor layer of the photovoltaic element by the plasma CVD method. Particularly, it relates to a method of and an apparatus for mass producing photovoltaic elements such as solar cells using a roll to roll apparatus.
2. Related Background Art
There are various means for improving the photoelectric conversion efficiency of a photovoltaic element using a non-crystalline semiconductor, and it is necessary to improve the characteristics of a p type semiconductor layer, an i type semiconductor layer, an n type semiconductor layer, a transparent electrode, a back electrode, etc. constituting a photovoltaic element using the pin type semiconductor junction.
Particularly, regarding a so-called doping layer such as a p type semiconductor layer or an n type semiconductor layer, it is first required that the density of an activated acceptor or a donor be high and activating energy be small. Thereby, the diffusing potential (built-in potential) when the pin junction is formed becomes great and the open voltage (Voc) of the photovoltaic element becomes great, and photoelectric conversion efficiency is improved.
Next, the doping layer basically does not contribute to the creation of a photocurrent and therefore, it is required that this layer not hinder the incidence of light onto the i type semiconductor layer creating a photocurrent. In order to reduce the absorption by the doping layer, it is important to make the optical band gap wide and make the film thickness of the doping layer small.
As the material of the doping layer having the characteristics as described above, mention may be made of group IV semiconductor material such as Si, SiC, SiN or SiO. Materials in a non-crystalline or microcrystalline form have been studied.
Above all, group IV semiconductor alloy materials having a great band gap have been considered to be suitable because of their small absorption coefficient, and microcrystalline or polycrystalline semiconductor materials have been considered to be suitable because of their small absorption coefficient and small activating energy.
On the other hand, it is required that the interfacial level on the junction interface of homogeneous or heterogeneous junction formed between the doping layer and the i type semiconductor layer be small.
However, the grating consistency between the i type semiconductor layer and the microcrystalline or polycrystalline p type semiconductor layer is not good and therefore, a junction interfacial level is created.
Therefore, a significant reduction in the running property of carries and fill factor (FF) exists and improvement therein has become a task.
As a method of improving mass productivity, a continuous plasma CVD method adopting a roll to roll system is disclosed in U.S. Pat. No. 4,400,409.
According to this method, with a long belt-like member as a substrate, the substrate is continuously conveyed in the lengthwise direction while electrically conductive type semiconductor layers required in a plurality of glow discharging areas are accumulated and formed. An element having semiconductor junction can thus be continuously formed.
FIG. 8 of the accompanying drawings is a schematic view of a typical plasma CVD apparatus of the roll to roll type for successively laminating n, i and p type semiconductor layers to thereby form a photovoltaic element of the single cell type.
The reference numeral 801 designates the whole of an accumulated film forming apparatus. The reference numeral 802 denotes a long electrically conductive magnetic material belt-like member, the reference numeral 803 designates a pay-away chamber for the belt-like member, the reference numeral 804 denotes a take-up chamber for the belt-like member, and the reference numerals 805 to 807 designate accumulated film forming chambers, the reference numeral 805 denoting a chamber for forming an n type layer, the reference numeral 806 designating a chamber for forming an i type layer, and the reference numeral 807 denoting a chamber for forming a p type layer. The reference numeral 809 designates a discharge space. The reference numeral 808 denotes a gas gate, and the reference numerals 810 and 811 designate bobbins.
The procedure of forming semiconductor film will hereinafter be described with reference to FIG. 8.
The accumulated film forming apparatus 801 has the pay-away chamber 803 for the belt-like member 802 and the take-up chamber 804 for the belt-like member 802 disposed at the opposite ends thereof. The accumulated plasma CVD film forming chambers 805, 806 and 807 by the plasma for forming a plurality of semiconductor layers are connected in series through the gas gate 808 between the pay-away chamber and the take-up chamber. Scavenging gas such as H2 gas is introduced into the gas gate 808 and forms a pressure barrier relative to the accumulated film forming chambers at the opposite ends, and the diffusion of the gas between the chambers can be prevented, and this forms a feature of the roll to roll type film forming apparatus. A material gas is supplied to each accumulated film forming chamber, and discharge can be caused in the discharge space 809 by the inputting of high frequency wave or microwave electric power.
Also, each accumulated film forming chamber has exhaust means and a pressure regulating valve and can be maintained in a reduced pressure state of predetermined pressure.
In actual film formation, the long belt-like member 802 is paid away from the pay-away chamber 803 and is passed over the pay-away chamber 804, and semiconductor layers can be successively accumulated and formed in the discharging space of the accumulated film forming chambers 805, 806 and 807 while the belt-like member 802 is continuously paid away and moved.
Also, a photovoltaic element of the tandem cell type can be made by adopting a chamber construction in which the n, i and p type layer forming chambers are repetitively arranged.
So, the present invention has as its object to provide a photovoltaic element in which the junction interface between a non-crystalline i type layer and a microcrystalline electrically conductive type layer has good grating consistency and which has an excellent current-voltage characteristic and excellent photoelectric conversion efficiency by the use of a roll to roll apparatus for improving mass productivity, and a method of and an apparatus for continuously mass-producing such photovoltaic elements.
The photovoltaic element of the present invention is a photovoltaic element comprised of a semiconductor-junctioned element, characterized in that the element includes a first electrically conductive type semiconductor layer, a non-crystalline i type semiconductor layer, a microcrystalline i type semiconductor layer and a second electrically conductive type semiconductor layer comprising microcrystal, and is pin-junctioned.
The photovoltaic element of the present invention is characterized in that the semiconductor layers thereof are formed chiefly of silicon, and the non-crystalline i type semiconductor layer includes germanium. Also, the photovoltaic element of the present invention is characterized in that the element is constructed so as to have a plurality of pin junctions.
Also, the photovoltaic element of the present invention is characterized in that the second electrically conductive type semiconductor layer is constructed so as to be located on the light incidence side.
Also, it is characterized in that the second electrically conductive type semiconductor layer is a p type layer.
Also, it is preferable that the layer thickness of the microcrystalline i type semiconductor layer be 50 to 100 xc3x85.
Also, it is preferable that the layer thickness of the microcrystalline p type semiconductor layer be 80 to 150 xc3x85.
Also, it is characterized in that the impurity density of the microcrystalline p type semiconductor layer is 1021 atoms/cm3 or greater on the outermost surface thereof, and this impurity density decreases toward the microcrystalline i type semiconductor layer.
Also, it is preferable that the microcrystalline i type semiconductor layer be constructed so that an area in which the atomic density thereof is 1018 atoms/cm3 or less may have a thickness of at least 30 xc3x85.
Next, the method of manufacturing the photovoltaic element of the present invention is characterized by forming a first electrically conductive type semiconductor layer on a long substrate, forming a non-crystalline i type semiconductor layer thereon, forming a microcrystalline i type semiconductor layer thereon by the high frequency plasma CVD method, and forming a second electrically conductive type semiconductor layer comprising microcrystals thereon by the high frequency plasma CVD method.
Also, it is preferable that the formation of the microcrystalline i type semiconductor layer be done with SiH4 and H2 as raw material gases, the amount of supply of H2 to SiH4 be 50 times or greater, and the magnitude of high frequency electric power applied to the raw material gases be 0.2 W/cm2 or greater.
Also, it is preferable that the formation of the microcrystalline p type semiconductor layer be done with SiH4, H2 and BF3 as raw material gases, the amount of supply of H2 to SiH4 be 50 times or greater, the amount of supply of BF3 to SiH4 be 10 to 50%, and the magnitude of high frequency electric power applied to the raw material gases be 0.01 to 0.03 W/cm2.
Also, it is preferable that the formation temperature of the microcrystalline i type semiconductor layer be below the formation temperature of the non-crystalline i type semiconductor layer, and the formation temperature of the microcrystalline i type semiconductor layer be 180 to 240xc2x0 C.
Also, it is characterized in that the non-crystalline i type semiconductor layer be formed by the microwave plasma CVD method.
Also, it is characterized in that the non-crystalline i type semiconductor layer have an i type layer formed by the microwave plasma CVD method, and an i type layer formed by the high frequency plasma CVD method.
Further, one form of the apparatus for manufacturing the photovoltaic element of the present invention is an accumulated film forming apparatus for continuously accumulating a plurality of semiconductor layers on a long substrate by the plasma CVD method, characterized by at least a first accumulating chamber having means for letting a raw material gas flow from the upper part toward the lower part in the direction of movement of the long substrate, and a second accumulating chamber having means for letting the raw material gas from the lower part toward the upper part in the direction of movement of the long substrate, the first accumulating chamber and the second accumulating chamber being connected together by a separating path.
Also, it is characterized in that the area of at least an electrode in the second accumulating chamber for applying electric power for creating plasma is larger than the area of the long substrate in the accumulating chamber.
Also, it is characterized in that the electrode is of a fin-like shape or an enclosure-like shape.
Also, it is characterized in that the potential of the electrode is positive relative to the long substrate.
Also, it is characterized in that a portion for supplying the raw material gas into the accumulating chambers has a member for shielding the long substrate from the flow of the raw material gas.
Another form of the apparatus for manufacturing the photovoltaic element of the present invention is an accumulated film forming apparatus for forming accumulated film by the plasma CVD method, characterized in that the area of an electrode for applying electric power for creating plasma is larger than the area of a substrate in an accumulating chamber.
Also, it is characterized in that the electrode is of a fin-like shape or an enclosure-like shape.
Also, it is characterized in that the potential of the electrode is positive relative to the long substrate.
Also, it is characterized in that a portion for supplying the raw material gas into the accumulating chamber has a member for shielding the long substrate from the flow of the raw material gas.